Originally Posted by ICANHITHIMMAN
lowers are not caliber specific it looks like an ar 15 pattern lower so anything that is the same length as a 5.56 nato round could work. That said the PRICE IS F ING CRAZY HIGH!!!!!!!!
Actually, yes, AR's CAN be caliber specific, if so stamped. Most are stamped 223/5.56 - so, legally, have to build that caliber.
Some lowers are marked rifle or pistol.
Some lowers are marked "Multi Cal" - can build what you want.
Also, depends on how the lowers are entered into the FFL's book - depends on what you can build.